1. Field of the Invention
The present invention relates to a recording apparatus, and more specifically to a recording apparatus which uses a stepping motor for feeding a recording medium.
2. Discussion of the Prior Art
A recording apparatus is known, wherein an image is formed on a recording medium, by a multiplicity of parallel recording lines each line defined by a straight row of picture elements which are the smallest parts of the image. This type of recording apparatus includes (a) recording means for effecting a recording operation along each recording line on the recording medium, according to recording data which include bits corresponding to the picture elements arranged in a straight row, (b) feeding means for feeding the recording medium in a feeding direction perpendicular to the recording line, (c) signal generating means for generating a timing pulse at a point of time which has a predetermined time relation with a point of time at which the recording means starts recording along the recording line, and (d) recording control means responsive to the timing pulse, for activating the recording means to start recording along the recording line.
A laser printer is an example of a recording apparatus of the type indicated above. The laser printer has a photoconductive drum as a recording medium, a laser source for generating a laser beam, and a polygon mirror for reflecting and deflecting the laser beam over a predetermined angular range, so that local areas of the surface of the photoconductive drum corresponding to the picture elements of each recording line are scan-exposed to the deflected laser beam. The photoconductive drum is rotated by a predetermined incremental angle during the scan-exposure of each recording line. Thus, a line of latent image represented by the laser beam is formed along each recording line on the surface of the photoconductive drum. This latent image is developed into a corresponding visible image such that a toner is deposited on the selected local areas of the drum surface according to the pattern of the latent image. The visible toner image is then transferred to another recording medium in the form of a cut sheet, for example. For justification of the recording lines at the beginning of recording, a generator is provided as the signal generating means, which is adapted to produce a synchronization pulse in response to which the scanning of each recording line is started to form a line of latent image. Thus, the point at which the image scanning or exposure is started is made constant for all the recording lines.
In a thermal line printer, a heat-sensitive paper containing an inking material which produces a color upon exposure to heat is used as a recording medium, and a multiplicity of heat-generating elements arranged in a straight row are selectively energized to effect printing along each recording line on the recording medium as the recording medium is fed. In this case, the selective energization of the heat-generating elements for each recording line is effected in response to the timing pulse in the form of a line printing command generated by the suitable signal generator.
Where a servomotor is used as a drive source of the feeding means, the recording medium can be fed at a constant speed, permitting a high quality imaging with the line spacing distance kept constant. Recently, however, the servomotor is increasingly replaced by a stepping motor, in view of a relatively high cost of the servomotor and a control system therefor.
On the other hand, the operating speed of a stepping motor tends to vary periodically, due to stepping of the motor from one excitation phase to another. This means an inevitable variation in the feeding speed of the recording medium. For simplification of the control device and reduced cost of the recording apparatus, the stepping motor is usually controlled in an open-loop manner. This open-loop control arrangement causes inconsistent timing of stepping of the stepping motor, also leading to a variation in the feeding speed of the recording medium. The variation in the feeding speed results in an undesirable fluctuation in the line spacing, which deteriorates the quality of an image recorded.
A closed-loop control arrangement using an encoder for stepping the stepping motor in response to a pulse signal generated by the encoder reduces the amount of variation in the feeding speed of the recording medium due to the inconsistent timing of stepping of the motor. However, the closed-loop control arrangement still suffers from the variation in the feeding speed due to the stepping of the motor itself from one excitation phase to the next, and is available at an increased cost as compared with the open-loop control arrangement.